Preserve AS2 content line endings

[m0a0k0s]

Hello. I'm migrating legacy code that handles AS2 messages to MimeKit.
Old version respects the original line ending for content that passes as binary data.
In MimeKit, there's two places: encrypting ApplicationPkcs7Mime.EncryptAsync and signing MultipartSigned.CreateAsync that forces line ending transformation to DOS.
It references rfc2015 or rfc3156, section 5.1 here, but it's for plain text

MimeKit/MimeKit/Cryptography/MultipartSigned.cs

Line 146 in 7042714

// Note: see rfc2015 or rfc3156, section 5.1

When creating the message as is, the mdn calculated on original content outside MimeKit does not match with the one calculated with the result message.
I bypass it now by calling the underlying code directly without conversion part.
This is the same as #569, but another direction - creating a message.

[jstedfast]

I'll try to look at this in the next few days.

[jstedfast]

Okay, so I notice that AS2 seems to not have a Content-Transfer-Encoding header (based on the similar issue you linked).

That makes AS2 non-standard MIME in that sense.

Is this how things need to be for compatibility or can you set the AS2 part's ContentTransferEncoding to Binary?

(Note: As of right now, even if you set the AS2 part's ContentTransferEncoding to Binary it will still fail for MultipartSigned but that is something that I am already fixing in MultipartSigned.Create).

What I need to know is whether setting that header will break compatibility. If it will, then obviously I will need to come up with a hack for this. BUT if you can safely set ContentTransferEncoding to Binary, then this is much much much easier to fix.

[jstedfast]

From RFC4130, section 5.2.1:

5.2.1.  Content-Transfer-Encoding Not Used in HTTP Transport

   HTTP can handle binary data and so there is no need to use the
   content transfer encodings of MIME [1]].  This difference is discussed
   in [3], Section 19.4.5.  However, a content transfer encoding value
   of binary or 8-bit is permissible but not required.  The absence of
   this header MUST NOT result in transaction failure.  Content transfer
   encoding of MIME bodyparts within the AS2 message body is also
   allowed.

Okay, so, it appears that it is safe to set Content-Transfer-Encoding to Binary.

Would you be ok with doing that?

[m0a0k0s]

yes, Content-Transfer-Encoding was already set to binary in my case